Subminiature fuses, like other types of fuses are used to protect circuit components from damage that can be caused by excess current flowing through the circuit. Excess current is generally categorized as either an overload current or a short circuit current. Overload current is generally considered to be in the range of 135 percent to 200 percent of normal or rated current. Short circuit current may be 1000 percent of rated current or greater.
Many conventional fuses are constructed from a fuse element and a two piece fuse housing comprising a cap and a base. During a short circuit condition, pressure inside the housing increases. Due to the small physical size of the subminiature fuse and hence the short arc clearing gap, the housing for such a fuse is subject to catastrophic failure problems that are not normally inherent in a physically larger fuse. There is a risk that the fuse housing will blow apart or rupture. In the two piece housing design, this normally occurs at the seal between the cap and the base. If the housing ruptures, this would not only expose a live arc but would also prolong that arc thereby potentially causing damage to circuit components downstream of the fuse due to the additional time required to fully interrupt the circuit.
Once the housing begins to leak, the pressure in the housing begins to decrease. This causes the interruption time to increase.
Those skilled in the art know that when the fuse element is subjected to short circuit current, the fuse element heats up until it reaches the melting point of the fuse element conductor. The rate of the heat build up is, among other things, a function of the magnitude of the excess current. Once the temperature of the conductor reaches its melting point, the conductor material rapidly vaporized mixing vaporized metal atoms with the gas or air medium surrounding the conductor. Upon vaporization an arc is formed in the gas mixture which is created by the vaporization of the fuse element. The resulting plasma acts as a conducting path for the arc. The increased temperature of the arc plasma also increases the pressure in the fuse housing. If the arc plasma becomes dense, the travel of the charged particles in the plasma is restricted. Decreased mobility of the charged particles increases the resistance of the gap, thereby acting to extinguish the arc.
There have been several attempts to solve this problem of catastrophic fuse failure in subminiature fuses. One example is illustrated in the U.S. Pat. No. 4,417,226 to Asdollahi, et. al. In this patent a ceramic lining is utilized in the interior of a two piece fuse housing to insulate the plastic body from the heat produced during a short circuit condition. Merely coating the interior of an air filled fuse housing with a ceramic lining does not provide a fast clearing fuse. The relatively large interior volume of air and a low out-gassing ceramic lining prevents the quick increase in pressure required for fast arc clearing. A lower pressure in the fuse housing tends to facilitate charged particle mobility in the plasma during interruption of an arc during a short circuit condition. This results in a longer arc time which results in a higher pressure metal rich gas. Such prolonged arcing raises the risk of catastrophic fuse failure.
Other examples of subminiature fuses are embodied in U.S. Pats. Nos. 2,941,059 to Sims, et. al. and 3,775,723 to Mamrick, et. al. Others have attempted, but have failed, to reduce this risk of catastrophic failure by improving the strength of the fuse housing to prevent rupture thereof.
Thus, there exists the need to provide a subminiature fuse capable of properly interrupting short circuit current with a minimal risk of catastrophic failure. A subminiature fuse device with a short duration arcing time would be widely received by the industry.